Conventional electrochemical fuel cells convert fuel and oxidant into electrical energy and a reaction product. A typical fuel cell comprises a plurality of layers, including an ion transfer membrane sandwiched between an anode and a cathode to form a membrane-electrode assembly, or MEA.
Sandwiching the membrane and electrode layers is an anode fluid flow field plate for conveying fluid fuel to the anode, and a cathode fluid flow field plate for conveying oxidant to the cathode and for removing reaction by-products. Fluid flow field plates are conventionally fabricated with fluid flow passages formed in a surface of the plate, such as grooves or channels in the surface presented to the porous electrodes.
A typical single cell of a proton exchange membrane fuel cell will, under normal operating conditions, provide an output voltage between 0.5 and 1.0 Volt. Many applications and electrical devices require high voltages for efficient operation. These elevated voltages are conventionally obtained by connecting individual cells in series to form a fuel cell stack.
To decrease the overall volume and weight of the stack, a bipolar plate arrangement is utilised to provide the anode fluid flow field plate for one cell, and the cathode fluid flow field plate for the adjacent cell. Suitable flow fields are provided on each side of the plate, carrying fuel (eg. hydrogen, or a hydrogen rich gas) on one side and oxidant (eg. air) on the other side. Bipolar plates are both gas impermeable and electrically conductive and thereby ensure efficient separation of reactant gases whilst providing an electrically conducting interconnect between cells.
Fluids are conventionally delivered to each fluid flow field plate by way of common manifolds that run down the height of the stack, formed from aligned apertures in each successive plate.
The area of a single fuel cell can vary from a few square centimetres to hundreds of square centimetres. A stack can consist of a few cells to hundreds of cells connected in series using bipolar plates.
Two current collector plates, one at each end of the complete stack of fuel cells, are used to provide connection to the external circuit.
The are a number of important considerations in assembling the fuel cell stack. Firstly, the individual layers or plates must be positioned correctly to ensure that gas flow channels and manifolds are in correct alignment.
Secondly, the contact pressure between adjacent plates is used to form gas tight seals between the various elements in the manifolds and gas flow channels. Conventionally, the gas tight seals include compressible gaskets that are situated on the surfaces of predetermined faces of the plates. Therefore, in order to ensure proper gas tight sealing, an appropriate compression force must be applied to all of the plates in the stack, orthogonal to the surface planes of the plates in the stack, to ensure that all gaskets and sealing surfaces are properly compressed.
Thirdly, a compressive force is essential to ensure good electrical connectivity between adjacent layers.
At the outer ends of the stack, substantially rigid end plates are usually deployed for the application of suitable compression forces to retain the stack in its assembled state.
A number of different mechanisms have been proposed which allow this compressive force to be applied and maintained.
Conventional fuel cell stacks, such as described in U.S. Pat. No. 3,134,697, deploy tie rods, which extend between two end plate assemblies, and pass through holes formed in the periphery of the end plates. These tie rods are commonly threaded and employ fastening nuts to exert and maintain a clamping force.
Alternative configurations, such as described in U.S. Pat. No. 6,057,053, use similar mechanisms but the tie rods pass through the central portion of the stack, and hence active cells, within fluid manifolds or conduits.
Hydraulic methods have been employed, such as described in U.S. Pat. No. 5,419,980, where a pressurised fluid is used to apply a compressive force to the fuel cells via an expandable bladder or balloon.
Clips, such as described in U.S. Pat. No. 5,686,200, and compression bands, such as described in U.S. Pat. No. 5,993,987, have also been proposed.
A disadvantage of existing plate compression systems is that multiple elements are generally required to effect the compression across the entire surface areas of the plates, resulting in a complex assembly technique to ensure that plate alignment and uniform compression across the plate surface are maintained during and after the assembly process.
It is an object of the present invention to provide a fuel cell stack assembly apparatus and method which are simple and cost effective to use. It is a further object of the present invention to provide a highly reliable, uniform compression to the plates in the stack.
The present invention provides a method for applying and retaining compression to the fuel cell stack through the use of a fixed carriage or framework into which the cells can be built directly.
According to one aspect, the present invention provides a fuel cell compression assembly, comprising:                a carriage unit having at least two opposing side walls maintained in spaced relation by a base member extending therebetween at a lower position on the sides,        the opposing side walls and base member thereby defining a cradle for receiving fuel cell plates,        the opposing side walls each including at least one engagement member on internal face for engaging with a top member forming the top of the carriage unit.        
According to a further aspect, the present invention provides a fuel cell compression assembly comprising:                a carriage unit cradle for receiving a stack of fuel cell plates and for maintaining the plates in substantially overlying relationship; and        a closure member adapted to close the carriage unit and apply pressure to the plates therein, by automatic locking engagement with the cradle when the closure member is brought into position with the cradle in a first direction substantially orthogonal to the plane of the plates.        
According to a further aspect, the present invention provides a method of forming a fuel cell stack comprising the steps of:                providing a carriage unit cradle for receiving a plurality of fuel cell plates into a confinement volume therein;        installing said fuel cell plates into the cradle to form a stack;        applying a carriage unit closure member to compress the fuel cell plates in a first direction substantially orthogonal to the plane of the plates and to engage the closure member with the cradle;        the carriage unit providing automatic locking engagement of the closure member and the cradle when the closure member has reached an appropriate degree of compression of the plates.        